BRACHIOPODA. 
251 
Diagnosis. Shells transversely subsemicircular or semielliptical; greatest 
width along the hinge-line. Surface concavo-convex and covered with fine 
radiating striae which are equal or alternate in size. The pedicle-valve is 
slightly convex about the umbo, but becomes rapidly concave toward the middle 
with the apex perforated, except in old age. The cardinal area is conspicuous 
and nearly vertical, and the delthyrium closed by a convex plate or deltidium. 
The teeth are widely divergent and are supported by plates which are produced 
into elevated ridges nearly surrounding the muscular area. The latter is 
relatively short, subcircular in outline, deeply excavated and divided medially 
by a more or less distinctly defined longitudinal ridge which is often continued 
over the pallial region. 
The brachial valve is concave at the umbo, becoming strongly convex with 
growth; it has a much narrower cardinal area and the delthyrium is rudiment¬ 
ary or incomplete. Dental sockets deep, and continued as narrow grooves or 
indentations across the cardinal area. The crural plates are extended laterally 
with a slight curve, but are not supported by septa; at their inner margins 
they unite to form a callosity, upon which rests the short, bilobed cardinal 
process, which scarcely extends beyond the hinge-line. The muscular surface 
of this process is cordate in outline and is placed at a low angle to the plane 
of the area. A low median ridge extends forward from the hinge-plate 
separating two large adductor scars, in front of which are two narrow elongate 
impressions. Vascular and ovarian markings frequently well defined. Shell- 
substance fibrous, strongly punctate. 
Type, Strophomena rugosa Rafinesque (de Blainville), l%25,=Leptcena planum- 
bona, Hall, 1847, as recognized by King and other authors. 
Limiting the term Strophomena by this strict construction, it will prove to 
be a genus of comparatively moderate vertical range, appearing so far as known 
not before the age of the Trenton ( Leptcena filitexta , Hall, Strophomena Thalia, 
Billings, S. fluduosa, Billings, S. subtenta, Conrad), and becoming quite abundant 
in the Hudson River group ( Leptcena planumbona, Hall, L. planoconvexa, Hall, 
Strophomena sinuata, Emmons, Hemipronites nutans, Meek, Strophomena Wisconsin- 
